The present invention relates to an improved process for preparing nuclear-fluorinated aromatics by a halogen exchange reaction (halex reaction) in the presence of a catalyst.
Nuclear-fluorinated aromatics are important intermediates for preparing biologically active substances for pharmaceutical and agrochemical applications.
It is known to carry out halex reactions in aprotic, strongly polar solvents using metal fluorides at elevated temperature and in the presence of alkylammonium or alkylphosphonium salts (U.S. Pat. No. 4,287,374), pyridinium salts (WO 87/04149), crown ethers (DE-A 197 02 282), or tetraamidophosphonium salts (WO 98/05610). Disadvantages in reactions of this type are, particularly when weakly activated aromatics are used, the high reaction temperatures and long reaction times that are required. This leads to high energy consumption and low space-time yields. The high reaction temperatures frequently lead to the formation of unwanted by-products and decomposition products. In addition large amounts of expensive solvents are required.
Furthermore, for example, the tetraamidophosphonium salts (WO 98/05610) are extremely toxic.
The requirement therefore still exists for a process for preparing nuclear-fluorinated aromatics by a halex reaction in which less energy is consumed, higher chemical and space-time yields are possible, and solvents may optionally be avoided.